(1) Field of the Invention
The present invention relates to a pressure difference detector for inspecting air tightness of a closed container in a pipe line system for an automobile engine, a method for judging an abnormality of the pressure detector, and a fuel/vapor emission preventing apparatus employing the detector.
(2) Description of the Related Art
FIG. 1 is a diagram showing a construction of a conventional fuel/vapor emission preventing apparatus. In the figure, reference numeral 1 is a fuel tank, 2 is a canister for absorbing fuel vapor generated from fuel in the fuel tank 1, and 3 is a communication tube connected between the fuel tank 1 and the canister 2. Reference numeral 4 is a purging pipe connected between the canister 2 and an intake pipe 6 communicated with an engine, 7 is an air valve for opening canister 2 to atmospheric air, 8 is a pressure difference detector having one end for introducing a pressure in the fuel tank 1 and another end open to atmospheric air, and 9 is a cap for sealing the fuel tank 1.
Next, an operation of the apparatus shown in FIG. 1 will be described. In the fuel tank 1, fuel and air are contained. The air part in the fuel tank 1 is communicated with atmospheric air through the canister 2 and through a check valve (not shown) mounted on the cap 9 so that, when the pressure in the fuel tank 1 is lowered, atmospheric air is introduced into the fuel tank 1, and when the pressure is increased, the air in the fuel tank is discharged to the atmosphere. Thus, the pressure in the fuel tank 1 is controlled to protect the fuel tank 1 against an excessive pressure.
Since the fuel generally includes a volatile component, however, the air in the fuel tank 1 includes fuel-vapor gas. Therefore, if the air including the fuel-vapor gas is discharged directly to the atmosphere, it causes contamination. To prevent the contamination of the air, conventionally, when the pressure in the fuel tank 1 is increased, the intake valve 5 is opened to introduce the air containing the fuel-vapor gas into the engine through the communication tube 3, the canister 2, the intake valve 5, and the intake pipe 6.
If, however, the air tightness of the fuel-vapor gas emission preventing apparatus is deteriorated due to the fact that, for example, the cap 9 is not completely closed, or some part of the fuel-vapor gas emission preventing apparatus is damaged, a large amount of fuel-vapor gas is leaked into the atmosphere. To ascertain the air tightness of this apparatus, while a vehicle is being driven, the output signal of the pressure difference detector 8 is monitored under the following pressure detectable conditions, i.e., while the engine is made to be in a high power outputting state and while the intake valve 5 is opened, the air valve 7 is closed to make the inside of the emission preventing apparatus including the fuel tank 1 and the canister 2 at a negative pressure, and then the intake valve 5 is closed to make the apparatus a closed chamber. In general, the pressure difference monitor 8 outputs a signal proportional to the change of the pressure. Therefore, under the condition that the fuel vapor gas emission preventing apparatus is closed, if a rapid change of the pressure is generated, the output of the pressure difference detector 8 is also largely changed. By detecting this large change of the output, leakage of the fuel vapor is detected and the preventing apparatus is judged to be malfunctioning. FIG. 2A shows the output signal of the pressure difference detector 8 when the fuel-vapor gas emission preventing apparatus is under a normal operation without leakage. As can be seen from the figure, the negative pressure is constant in this case. FIG. 2B shows an example of the output signal of the pressure difference detector 8 when there is a leakage in the fuel-vapor gas emission preventing apparatus so that it is abnormal. As can be seen from the figure, the negative pressure changes along with time to converge with the atmospheric pressure.
In the above-described conventional apparatus, there is a problem as follows. That is, when the air tightness between a reference pressure introducing side for introducing the atmospheric air and a measuring pressure introducing side for introducing the internal pressure of the fuel tank 1 becomes insufficient, if the atmospheric air leaks from the reference pressure introducing side to the measuring pressure introducing side, the output signal of the pressure difference detector 8 representing the negative pressure changes along with time as shown in FIG. 2C, even when the fuel-vapor gas emission preventing apparatus itself is quite normal so that there is no leakage in the fuel-vapor gas emission preventing apparatus except for the pressure difference detector 8. If the change of the output signal of the pressure difference detector 8 due to the leakage in the fuel vapor gas emission preventing apparatus shown in FIG. 2B is very similar to the change of the output signal of the pressure difference detector 8 due to the leakage in the pressure difference detector 8 shown in FIG. 2C, a problem arises in that, from the state shown in FIG. 2B and from the state shown in FIG. 2C, it is impossible to judge whether the malfunction resides in the fuel-vapor gas emission preventing apparatus or in the pressure difference detector 8.